The German plan, as worked out by Erich von Falkenhayn, chief of the general staff of the German army, was to attack the French fortress at Verdun in great strength in an effort to weaken the French irretrievably by causing the maximum possible number of casualties. The Allied plan for 1916, as laid out by commanders in chief, Marshal Joffre of the French army and General Sir Douglas Haig of the British, was to attempt to break through the German lines in the west by a massive offensive during the summer in the region of the River Somme. The Germans opened the Battle of Verdun, on February 21. After bitter fighting the Germans took Fort Douaumont (February 25), Fort Vaux (June 2), and the fortifications of Thiaumont (June 23), but did not succeed in capturing Verdun. (It was here that General Henri Philippe Pétain gained prominence as the "hero of Verdun".) Because of the severe losses in the battle, the French were able to contribute to the Allied offensive on the Somme only 16 divisions of the 40 originally planned; the offensive, which began on July 1 and continued until the middle of November, consequently was largely in the hands of the British. They succeeded in winning about 325 sq km (125 sq mi) of territory, but the drive did not bring about a breakthrough. The First Battle of the Somme marked the earliest use of the modern tank, deployed by the British on September 15 in an attack near Courcelette. From October to December the French staged a counter-attack at Verdun and succeeded in recapturing Forts Douaumont and Vaux (November 2), restoring the situation that had prevailed before February. In August Hindenburg replaced Falkenhayn as German chief of staff with General Erich Ludendorff. In December General Robert Georges Nivelle succeeded Joffre as commander in chief of the French armies in the north and north-east.